Commercially available propylene polymers such as propylene homopolymers, propylene copolymers containing 1 to 10 weight percent ethylene, 1-20 weight percent C.sub.4 -C.sub.10 .alpha.-olefin, and heterophasic propylene polymers or mixtures thereof, have relatively low melt strength. Thus, when molten, propylene polymers exhibit little or no strain hardening (an increase in resistance to stretching during elongation of the molten material). Low melt strength can lead to a variety of melt processing problems, including sheet sag and local thinning in melt thermoforming processes, and flow instabilities in the coextrusion of laminate structures.
High melt strength propylene polymers can be prepared from normally linear propylene polymers. U.S. Pat. No. 5,554,668 discloses the formation of free-end long branches by irradiating linear propylene polymer material in a substantially oxygen-free environment (less than about 5% by volume) with high energy radiation at a dose and for a time effective to produce a substantial amount of molecular chain scission but insufficient time to cause gelation of the propylene polymer material, maintaining the irradiated propylene polymer in the substantially oxygen-free environment for a time sufficient for a significant amount of chain branches to form, and then deactivating substantially all the free radicals present in the irradiated propylene polymer material. The resulting branched propylene polymer material has high melt strength.
Various proposals have been made to increase the adhesion of propylene polymer layers to substrates such as metal. Known surface treatments include corona discharge and chemical modification such as exposure to a concentrated solution of sulfuric acid and chromic acid. U.S. Pat. No. 4,824,736 discusses the addition of maleic anhydride-modified polypropylene exhibits good adhesion to metal. See also U.S. Pat. No. 5,441,999, which discloses a hot melt adhesive comprising a blend of a low molecular weight polyolefin, olefin-maleic anhydride grafted copolymer, a propylene-ethylene copolymer and a tackifing resin.
Commercially available propylene polymer materials, high melt strength propylene polymers and blends thereof do not significantly adhere to metal substrates, such as aluminum, even if subjected to post heating treatment, or to typical primer-coated polymeric substrates without post heating treatment. Thus, it is typically necessary to coat the metal substrates with a primer coating prior to applying the propylene polymer in order to ensure adequate adhesion and to post heat treat typical primer-coated polymeric substrates to achieve acceptable adhesion. Obviously, this type of surface pretreatment and/or post treatment introduces added complexity and cost to the manufacture of propylene polymer-coated substrates.
An object of the present invention is to provide a propylene polymer composition having high melt strength which has improved adhesion to metal without the need for a primer coating or to polymeric substrates, including lightly metallized polymeric substrates, with a primer coating without the need for post heating.
Another object of the present invention is to provide a substrate which has a layer of highly adherent propylene polymer on at least one surface.
A feature of this invention is the addition of modified propylene polymer grafted with an unsaturated compound having a polar group to a high melt strength propylene polymer composition to improve its adhesion to metal and plastic substrates.
An advantage of this invention is the elimination of a intermediate adhesive coating between the metallic substrate and the high melt strength propylene polymer composition.